1. Field of the Invention
The present invention relates to optical devices and, in particular, to a nonlinear optical logic device in which at least two input light beams having independent modes are launched, the device operating independently of the relative phase between the two inputs.
2. Background Art
Bistable optical devices wherein the intensity of the output optical beam may be in either a low or high state for a given optical input, depending on the prior history of the device, have been previously described in the prior art. The potential applications for such devices include use as optical memory element, limiters, switches and logic gates.
One such example of a prior art bistable optical device is the nonlinear Fabry-Perot interferometer or resonator. The bistable feature of this device is provided by feedback due to mirrors and by the nonlinear refractive index of the medium that fills the device cavity. Although the bistable nature of the device would lead one to conclude that it could be utilized as an optical logic device in a multiple device optical data processing system, the requirement that the two optical input beams be combined prior to launch into the resonator in turn requires that the input beams be in phase coherence in order for the device to perform its intended logic function. This latter requirement is extremely difficult to provide in such systems. Further, Fabry-Perot optical logic "OR" gates could provide problems in complex optical data systems since the output signal phase shift is dependent on the input data, the output "1" logical signal for two input "1s" being different from the output "1" logical signal for an optical input comprising a logical "1" and a logical "0." Compensation for this difference is very difficult.
The use of optical nonlinear ring resonators as filters and bistable devices has also been described. For example, an article by R. G. Walker et al., "Integrated Optical Ring Resonators Made by Silver Ion-Exchange in Glass," Applied Optics, Vol. 22, No. 7, pp. 1029-1031 (April, 1983), describes an integrated optical ring resonator fabricated using silver ion-exchanged waveguides and used as a wavelength-selective filter.
In an article by Haavisto et al., "Resonance Effects in Low Loss Ring Waveguides," Optical Letters, Vol. 5, No. 12, pp. 510-512 (December, 1980), a technique for fabricating a thin-film ring resonator is described. An article by Sarid, "Analysis of Bistability in a Ring Channel Waveguide," Optics Letters. Vol. 6, No. 11, pp. 552-3 (November 1981), discloses the bistable operation of a ring-channel waveguide that is coupled to two line channels.
Although the aforementioned articles illustratte that the bistable (on-off) characteristics of nonlinear ring resonators in response to a single light beam input is widely known, only recently has an attempt been made to configure a device such that two light beams could be applied thereto so that the device could be utilized as an optical logic gate. In particular, an article entitled "Components for Optical Logic," Electronics, pp. 31-32 (Dec. 29, 1982), discloses a bistable optical device comprising a nonlinear ring resonator and two waveguide-like channels. The device, a one-chip optical gate, launches an input light beam into the resonator by first combining two optical inputs in one of the waveguide channels. Although this chip provides significant improvements over prior devices, a factor in limiting its use in large optical data processing systems is the same factor that limits wide application of a Fabry-Perat optical logic gate; phase coherency is required between the two optical inputs.
What is therefore desired is to provide an optical device which is capable of providing logic functions in response to at least two optical signal inputs and wherein the device performance is insensitive to the phase coherency of the input signals. Preferably, the logical operation can be performed entirely with optical components.